This invention relates to an optoelectronic integrated circuit device for use in optical receiving units or others of receivers for the optical fiber communication.
A conventional optoelectronic integrated circuit device (hereinafter called OEIC array) for use in optical receiving units or others for optical fiber communication is disclosed in "High-speed eight-channel optoelectronic integrated receiver arrays comprising GainAs PIN PDs and AlInAs/GaInAs HEMTs", Optical Fiber Communication Conference, Tuesday, Feb. 19, 1991, Lecture No. Tub2. Its schematic circuit diagram is shown in FIG. 1.
This OEIC array 40 is provided by monolithically integrated eight channel units of an optical receiving circuit. Each channel unit includes a photodiode PD (PD.sub.1 .about.PD.sub.8) for converting a received optical signal to an electric signal, and an amplifier AMP.sub.1 -AMP.sub.8) having resistors, high electron mobility transistors or others for amplifying the electric signal.
In such conventional OEIC array 40, the electric power source nodes (contacts) 41 of the respective preamplifiers AMP are connected to one another and have the same potential. Because of such circuitry, it is a problem that crosstalk, i.e., leakage of a signal input to one channel to other channels, has a level as high as -30 dB.
A source for crosstalk may be as follows: condensers C.sub.1 and C.sub.2 are inserted in circuits inside and outside the OEIC array 40 for the reduction of an impedance of the AC current component, and all the electric power source circuits are grounded through the condensers C.sub.1, C.sub.2. On the other hand, an electric source power supply terminal 42 is usually supplied with a current from an electric power source 43 through a limited length of an electric power source wiring 44. This electric power source wiring 44 has an inductance L as a distributed constant. This inductance L and the condenser C.sub.2 resonate at a specific frequency, and at this frequency an impedance between the electric source power supply terminal 42 and the ground becomes infinite. When a signal is input to one channel, an output voltage corresponding to the signal is generated at the output terminal 45, and a signal current is supplied to a load 46. This signal current is supplied by the electric power source 43. At the time of the supply of the signal current from the electric power source 43, a voltage equal to a product of the impedance between the electric source power supply terminal 42 and the ground, and a value of a current flowing there is generated in the electric source power supply terminal 42.
Accordingly, at the above-described resonance frequency the impedance between the electric power source supply terminal 42 and the ground is infinite, and a very large voltage change or deflection corresponding to the input signal current occurs. Generally, the preamplifier AMP has a deflected output voltage corresponding to a deflection of an electric power source voltage. Consequently, at this resonance frequency, an electric signal input to one channel causes a large source voltage deflection which causes deflections of output voltages of the all the other channels through the common electric power source wiring. This is a cause for the occurrence of crosstalk.
FIG. 2 shows a detailed structure of an equivalent circuit of the OEIC array 40 in FIG. 1. In this OEIC array, the first channel unit of the eight channel units includes a PIN photodiode PD.sub.1, and an amplifier AMP, whose components include resistors R.sub.11 .about.R.sub.14, and high electron mobility transistors (HEMTs) Q.sub.11 .about.Q.sub.15. The transistor Q.sub.11 and the resistor R.sub.12 constitute an amplification stage. The transistors Q.sub.12 .about.Q.sub.15, and the resistors R.sub.13, R.sub.14 constitute a two-stage source follower. The remaining channel units have the same structure as the first channel.
Another example of the conventional OEIC arrays of this kind is disclosed in "Monolithic Integration of an InP/InGaAs Four-Channel Transimpedance Optical Receiver Array", Electronics Letters, vol. 26, pp. 1833.about.1834, October 1990. FIG. 3 schematically shows its circuit diagram.
In this OEIC array, four channel unit constituting the optical receiver are monolithically integrated. A first one of the four channels comprises a PIN photodiode PD.sub.1, an input resistor device R.sub.f1, junction transistors Q.sub.11 .about.Q.sub.19, and level shift diodes LSD.sub.1. The transistors Q.sub.11 .about.Q.sub.13 constitute an amplification stage for amplifying an electric signal photo-electrically converted by the photodiode PD.sub.1. The transistors Q.sub.14, Q.sub.15, and the diodes LSD.sub.1 constitute a level shifting stage for shifting a d.c. voltage of an amplified signal outputted by the amplifying stage. The transistors Q.sub.16 .about.Q.sub.19 constitute a source follower stage for securing a fan out of an optical receiver output O/P.sub.1. The remaining channel units have the same circuitry as the first channel unit.
In a conventional OEIC array of FIG. 3, the amplification stages and the level shift stages of the respective preamplifiers are supplied with an electric source power V.sub.DD1 through an electric power source wiring which is common to the respective channels. The source follower stages are supplied, with an electric power source V.sub.DD2 through a different electric power source wiring which is common to the respective channels. In a conventional OEIC array of FIG. 2, all the stages of the respective preamplifiers are supplied with an electric source power V.sub.DD through an electric power source wiring which is common to the respective channels. In the conventional OEIC arrays of these structures, large modulated currents are caused by input signals in the level shift stages and the source follower stages as will be described below. Consequently in the electric power supply sources for these stages, large source voltage deflections are caused by the modulated currents. Such source voltage deflections affect the common electric power sources of the respective channels connected to these stages. That is, a source voltage deflection caused by an input signal in one channel affects the power sources of the other channels, and causes voltage deflections in the output terminals of the other channels. Consequently crosstalk takes place among the channels. As a result, problem crosstalk deteriorates minimum sensitivities of the optical receiving circuits of the respective channels.
With respect to the OEIC array of FIG. 2 using HEMTs, explanation will be made as follows with reference to FIG. 4. Here it is assumed, for example, that an optical signal input to the first channel is converted to an electrical signal by a photodiode P.sub.D1, and a modulated current .DELTA.I.sub.ml is caused by an input signal when the electric signal is amplified by the preamplifier consisting of an amplification stage and a follower stage. 90% of a modulated current .DELTA.I.sub.ml is generated in the source follower state. This modulated current .DELTA.I.sub.ml flows through the wiring having a resistance R.sub.p to thereby generate a voltage deflection .DELTA.V.sub.D. This wiring resistance R.sub.p is equivalently shown at the electric power source terminal V.sub.DD for supplying an electric source power commonly to the respective preamplifiers. The above-described modulated current .DELTA.I.sub.ml flows through this wiring resistor and generates a voltage deflection .DELTA.V.sub.D (=.DELTA.I.sub.ml xR.sub.p). This voltage deflection .DELTA.V.sub.D is source voltage deflections of the preamplifiers of the other channels as it is. Generally when a source voltage is deflected, an output voltage of the preamplifier is deflected. Accordingly, when an optical signal is input to one channel, source voltages of the preamplifiers of the other channels are deflected, and the deflected source voltages deflect output voltage of the other channels with a resulting in crosstalk.
In the OEIC array of FIG. 3 using junction transistors, modulated currents are generated due to input signals. 90% of a modulated current is generated in the level shift stage or the source follower stage. The amplification stage and the level shift stage of each preamplifier are supplied with a source voltage V.sub.DD1 through the common electric power source wiring. Consequently, when a modulated current occurs in the level shift stage of one preamplifier, a voltage deflection .DELTA.V.sub.D is generated in the common electric power source wiring as described above. This voltage deflection .DELTA.V.sub.D deflects source voltages of the preamplifiers of the other channels through the common power source wiring, and deflects output voltages. That is, an optical signal input to one channel deflects output voltages of the other channels, and crosstalk takes place.
The reason for output voltages being deflected when source voltages of the preamplifiers are deflected is as follows. That is, as shown in FIG. 4, each preamplifier includes an amplification stage, a level shift stage, a source follower stage. Output voltages of the level shift stage and of the source follower stage are not substantially deflected. But an output voltage of the amplification stage is deflected in a range from 50% to 100% and is sensitive to the voltage deflections. Accordingly, when a voltage deflection due to a source voltage deflection takes place in an output of the amplifier of one channel, this voltage deflection directly affects the source follower stages and the level shift stages. That is, when a source voltage of a preamplifier is deflected, output voltages are adversely deflected.
Because of crosstalk due to this cause, as described above, the minimum receiver sensitivity of the optical receiving circuit of each channel deteriorates.
The above-described modulated current .DELTA.I.sub.ml is bypassed to the ground by a condenser monolithically integrated on the same semiconductor chip. But the bypass condenser integrated on the semiconductor chip can only have a limited capacity, and it is impossible to give the condenser a sufficiently large capacity. Accordingly all of a generated modulated current cannot be bypassed, and only a part of the current can be bypassed. Consequently a residual modulated current which has not been able to be removed is a cause for the occurrence of the above-described crosstalk. On the other hand, it makes the size of a semiconductor chip impractically large to increase a capacity of the condenser so as to remove all of a modulated current.